Digital terrestrial broadcasting started as a ground-based broadcasting service to be employed along with analog terrestrial broadcasting. BS (Broadcasting Satellite) digital broadcasting and CS (Communications Satellite) digital broadcasting were adopted as broadcasting services using satellites. When it is desired that both of the services be received in viewers' houses, each of the houses is required to be equipped with a television terminal that includes respective individual receivers or receiving functions for receiving these broadcasting services.
Meanwhile, broadband service, as represented by FTTH (Fiber to the Home), has also started full-fledged operation, and a method is being discussed to use FTTH to perform IP multicasts in order to perform IP redistribution of digital terrestrial broadcasting as a method to replace analog terrestrial broadcasting in areas where digital terrestrial broadcasting signals cannot be received. There has also begun to be discussion of the provision of a function of receiving IP redistribution to television terminals. Further, a service has been developed in which broadband service is used to allow users to view the latest Hollywood movies or the like in the form of VOD (Video on Demand) by using dedicated receivers.
Different delivery methods and delivery media have been adopted in services that have developed in various ways, such as digital terrestrial broadcasting, BS digital broadcasting, CS digital broadcasting, IP broadcasting (IPTV: Internet Protocol Television), VOD, and the like. This makes wiring for television terminals in viewers' houses and operations of such terminals complicated when plural services are to be used. Thus, a unified method of wiring and viewing is desired by users.
FIG. 1 illustrates a configuration of conventional IP redistribution in digital terrestrial broadcasting. An IPTV delivery station 101 includes a plurality of IP redistribution devices 111. An RF (Radio Frequency) receiver unit 113 in each of the IP redistribution devices 111 receives digital terrestrial broadcast waves input via an antenna 112, targeting IP redistribution.
As illustrated in FIG. 2, the above broadcast waves include digital information in the MPEG2-TS (Moving Picture Experts Group phase 2-Transport Stream) format. MPEG-2-TS is a format standardized by ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-1, and ITU-T (International Telecommunication Union Telecommunication Standardization Sector) standard H.222.0.
The RF receiver unit 113 outputs received MPEG2-TS-based image content portions and additional information portions to a CAS (Conditional Access System) cancellation unit 114 and an NIT (Network Information Table) conversion unit 117, respectively.
The CAS cancellation unit 114 cancels the CAS added to an image content portion so that conditional accesses are provided. A transcoding unit 115 transcodes the coding method from MPEG2 into 264/AVC (Advanced Video Coding). A CAS adding unit 116 again adds, to the image content portion, the CAS for IP redistribution. The NIT conversion unit 117 converts the NIT in the additional information portion into a table for IP redistribution. Thereby, frequency information in the NIT is converted into IP address information.
Thus, MPEG2-TS142 including a transcoded image content portion and an additional information portion is generated from received MPEG2-TS141, as illustrated in FIG. 3. An IP packetizing unit 118 packetizes the generated MPEG2-TS into an IP packet in the multicast format as illustrated in FIG. 4, and outputs the packet to an IP network 102.
In the IP network 102, multicast packets of all channels to be IP redistributed are relayed to a layer 2 switch (L2SW) 122 closest to an optical fiber 123 that accomodates a viewer.
A multicast router 121 disposed in the IP network 102 performs control in channel selection by exchanging messages M1 and M2 with an IP-redistribution-compatible terminal device 132 in a viewer's house 103. The exchange of messages is performed in accordance with IGMP (Internet Group Management Protocol), which is a multicast compatible group management protocol. Examples of the terminal device 132 include an IP-redistribution-compatible television receiver or recorder. As IGMP, IGMPv1 (RFC (Request For Comment) 1112) or IGMPv2 (RFC 2236) are used (see non-Patent Document 1 for example).
The layer 2 switch 122 monitors IGMP message M1 by using an IGMP snooping function, and outputs only packets addressed to a multicast group selected by a user to the optical fiber 123 accomodating that viewer.
In the viewer's house 103, multicast packets are transferred to the terminal device 132 from an ONU (Optical Network Unit) 131 connected to the optical fiber 123 in order to allow the viewer to view desired content through IP redistribution.
FIG. 5 illustrates the IP multicast relay system disclosed by Patent Document 1 below. A multicast server 161 in an IPTV delivery station 151 outputs multicast packets as illustrated in FIG. 6, and a center output device 162 relays a plurality of particular multicast packets. Thereby, a plurality of particular multicast packets in the format illustrated in FIG. 7 use different frequencies in order to be bypassed by CATV (Community Antenna Television) transmission lines 152.
A communication device 171 in a viewer's house 154 includes a table 176 registering the correspondence between frequencies and the addresses of multicast groups. When a user has selected a multicast group in IGMP message M3 using a multicast compatible terminal device 173, an IGMP receiver unit 178 receives that message and transfers it to a control unit 177.
The control unit 177 retrieves from the table 176 the frequency corresponding to the address of the selected multicast group, and transmits it to an RF receiver unit 174. The RF receiver unit 174 selects that frequency to receive the multicast packet, and a LAN (Local Area Network) output control unit 175 transfers a multicast packet as illustrated in FIG. 8 to the terminal device 173 via a LAN. Thereby, only packets for a particular multicast group are relayed to the terminal 173.
An NU (Network Unit) 172 in the viewer's house 154 is connected to the IPTV delivery station 151 via an IP network 153.
Next, an explanation will be given for management of communication resources (band resources) performed by an IGMP message on an IEEE (Institute of Electrical and Electronic Engineers) 1394 bus.
IGMPv2 defines four types of IGMP message, “Membership Report”, “Leave Group”, “General Query”, and “Group-Specific Query”.
Among them, “Membership Report” is a join message output from a terminal device for requesting that the terminal device be allowed to join a multicast group. “Leave Group” is a leave message output from a terminal device for requesting that the terminal device be allowed to leave a multicast group.
“Group-Specific Query” is a first query message output from a query router (querier) that has received “Leave Group” so that the query router can confirm whether or not there is another terminal device that participates in the same multicast group. “General Query” is a second query message periodically output from a query router in order to confirm whether or not there is a terminal device that participates each of all the multicast groups.
FIG. 9 illustrates a flowchart for a process performed when a terminal device joins a multicast group, disclosed by Patent Document 2. When receiving a join message for joining a multicast group transmitted from a terminal device (step 181), an IGMP router checks whether or not a service has started for the corresponding IP multicast address (step 182). When a service has not started, the IGMP router follows procedures for making the terminal device join the IP multicast address (step 183), and checks whether or not the procedures succeeded (step 184). When the procedures have failed, this fact is reported to the terminal device (step 189).
When the joining procedures succeeded, the IGMP router secures a synchronization channel number (step 185), and writes information about this IP multicast flow to the layer 3 flow register of the IGMP router itself (step 186). In writing this information, a counter value “1” is written to the connection counter in the layer 3 flow register. Thereafter, the same information is written to the layer 3 flow register in the terminal device (step 187).
When the service for the requested IP multicast address has started in step 182, the IGMP router increments the connection counter of its own layer 3 flow register (step 188).
As described above, when receiving a join message from a terminal device, a synchronization channel number is secured if the join message is requesting joining to a new multicast group, and if the join message is requesting joining to a multicast group for which a process has already started, the connection counter is incremented.
FIG. 10 illustrates a flowchart for a process performed when a terminal device leaves a multicast group, disclosed by Patent Document 2. When receiving a message for leaving an IP multicast address “IPm” from a terminal device (step 191), an IGMP router decrements the connection counter in its own layer 3 flow register (step 193).
The connection counter is also decremented (step 193) when a particular time has elapsed without receiving an IGMP message reporting that IP multicast address “IPm” is being received continuously (step 192). This corresponds to a case when there have been no responses to a second query message from any terminal devices within a particular period of time.
Next, the IGMP router checks whether or not the counter value of the connection counter has become “0” (step 194), and when the value has become “0”, it performs a process of leaving the IP multicast group (step 195).
Patent Document 3 discloses a digital broadcast receiver device for receiving digital broadcast information from a network. This digital broadcast receiver device includes means for extracting, from received digital broadcast information, digital broadcast information corresponding to a reception request made by a terminal device, and means for transcoding the extracted digital broadcast information into information in a format that allows transfer to the terminal device through a network so that the transcoded digital broadcast information is transferred to the terminal device.
Patent Document 1: Japanese Laid-open Patent Publication No. 2003-204355
Patent Document 2: Japanese Laid-open Patent Publication No. H10-308759
Patent Document 3: Japanese Laid-open Patent Publication No. 2001-094519
Non-Patent Document 1: “Internet Group Management Protocol, Version 2”, [online], [Searched for on Apr. 9, 2007], <URL:http://www.geocities.jp/kotekoteland/rfc/rfc2236j.txt> on the Internet